The ultimate properties and quality of blown film can be adversely affected particularly by cyclic variations in operating conditions, and by less than perfect regularity and consistency as regards the extruded resin. For example the temperature of the extrusion apparatus tends to have inherent cyclic character. Also cyclic conditions occur in conventional film cooling systems, whether based on a refrigerated source or if taken from ambient air. Additionally, film extruders frequently require a filtering system that will gradually clog with impurities, thus inducing a variable effect, particularly a change in melt index, in the resin passed through the filter. The resin itself may not be entirely consistant in quality, such as its melt index value and melt temperature.
Operating inconsistencies such of the above type can produce poor film in the sense of poor film flatness (i.e. appearance of wrinkles in the film) and poor uniformity of the gauge profile of the film. The gauge profile can be somewhat controlled by thickness measuring devices and systems, which give the operator some indication and warning when profile control is deteriorating, so that the appropriate control corrections can be made. However, when conditions go awry causing wrinkles to appear, there have not been good devices to predict this approaching condition, and to give the operator adequate warning to make appropriate corrections.
Thus even with close attention by a skilled operator, it has been difficult to control film quality above certain ceiling production rates. Even when operating within a production range considered manageable by a skilled operator, the film quality can be less than desired and less than specification tolerances, due to imprecise and inadequate control over cyclic and/or fluctuating operating conditions.
Accordingly, it would be advantageous to the art if there were available a blown film process which achieved finer and more precise control over film quality and/or properties.
It would be particularly advantageous if such a process provided a sufficient automatic counter-balancing effect to fluctuating and/or cyclic operating conditions to permit a significant increase in the possible rate of extrusion, while retaining an acceptable level of quality in the film produced, and/or which would consistently produce better quality film.